To combat the eavesdropping threats, physical layer security has been widely investigated in conventional wireless communications and also actively explored in satellite and aerial networks recently. As a supplement of cryptography protocol in the upper layer, physical layer security approaches are aimed to ensure secrecy capacity by exploiting the randomness difference of wireless channels[1]}. Particularly, several related works of physical layer security in satellite-terrestrial communications have been reported. To achieve the secure transmissions of satellite communications, additional communication facilities generally work as an assistance to enhance the main channel capacity or degrade the eavesdropping channel capacity. By exploiting terrestrial base stations (BSs) to serve as cooperative relays of satellite communications, an opportunistic user-relay selection criteria is proposed to improve the secrecy performance in hybrid satellite-terrestrial relay networks [2]}.
An unmanned aerial vehicle (UAV) relay and an aerial Eve are particularly considered in hybrid satellite-terrestrial networks, a 3D mobile UAV relaying strategy is designed and the secrecy capacity of satellite-terrestrial communications is analyzed in [3]}. A multi-user cooperation scheme is proposed to improve the secrecy rate of satellite communications, where the inter-user-interference serves as the green interference to degrade the Eves[4]}. In addition, the mutual interference between satellite and terrestrial networks generally degrades the system performance [5]},
however the green interference from terrestrial network can be designed by beamforming (BF) optimization to minimize the total transmission power while guaranteeing the secrecy rate constraint of satellite users and the common quality of service of terrestrial users[6]}. However, aforementioned related works in satellite-terrestrial networks only consider the security of satellite or terrestrial link independently and generally external resources are consumed to assist with the concerned secure link.
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